Theoretically, it is possible to drive inverters independently in all capacity ranges. However, when an environment which is not smaller than hundreds of kW is designed for a method for driving inverters independently, usefulness of the design is significantly lowered due to various issues including the limit to power semiconductor devices and restrictions by apparatus design. Therefore, to overcome the issues, inverters which have a capacity higher than a given value are conventionally designed in a method for driving a plurality of inverters with a capacity smaller than a design capacity in parallel.
Driving a plurality of inverters in parallel may be controlled in various methods. The most common method among the methods is to install a single master controller and a plurality of slave controllers to control the whole system through communication between them.
However, if this method is used, output current unbalance may occur between the inverters, which may be caused by different variables, for example, time differences that may occur by communication between the master controller and the slave controllers, the difference in power semiconductor features for the respective inverters, and time differences that occur between power semiconductor driving circuits installed in each inverter.
That is to say, although N inverters are driven with the same 3-phase PWM (Pulse Width Modulation) signals, a small difference may occur between output currents of each inverter Iu, Iv and Iw. As a result, it is essential to consider a sufficient reserve capacity when designing a system for driving inverters in parallel.
FIG. 1 is an exemplary graph illustrating the state of inverter output current unbalance in a conventional system for driving inverters in parallel.
Referring to FIG. 1, a small phase difference is shown among output currents Iu1, Iu2 and IuN of the N inverters. Because of such a difference, the sum of each inverter output does not match the whole system output in a system for driving inverters in parallel. For example, when designing power supply of 600 kW with three inverters, it is essential to design power supply by connecting three inverters of a capacity even bigger than 200 kW in parallel for safety reasons.
That is, it is essential to consider the derating factor K for driving the inverters in parallel. In particular, as more inverters are connected in parallel, K becomes smaller. Therefore, inverters of even bigger capacity than the capacity of inverters theoretically calculated are required. This is expressed with the following Equation 1.Ptot=K×N×Pinv  <Equation 1>where Ptot is total output power; K is a derating factor, 0<K≦1 (in inverse proportion to the number N of inverters connected in parallel; and Pinv is the capacity of each inverter connected in parallel. Therefore, there is a need to provide a new method for designing and operating a more efficient system by correcting the aforementioned output current unbalance of inverters driven in parallel.